The present invention relates to an improved edge dam for use with applicators of the trailing blade type for applying a coating liquid to a moving web of paper.
Conventional applicators of the trailing blade type include means for applying coating liquid to a paper web that is usually supported and carried by a backing roll. Such applicators include a chamber having an opening extending across and parallel to the web, together with a doctor blade at a trailing side of the opening for leveling the coating and a front wall or orifice plate extending from a leading side of the opening toward the web and defining an orifice for exit of excess coating material from an application zone between the wall and doctor blade. Coating liquid is supplied to the chamber and thence through the chamber opening into the application zone, and to seal end spaces between the front wall and doctor blade at side ends of the zone to prevent escape of coating material laterally of the web, edge dams are provided thereat.
For applicators of the foregoing types, the edge dam often comprises a felt, plastic or metal material configured to fill a somewhat triangular shaped opening at either end of the application zone defined by the doctor blade, orifice plate and backing roll. Ideally, the edge dam absolutely prevents leakage of any coating liquid laterally to exterior of the zone, whereby uncoated strips may be maintained along the side edges of the paper web. However, that can require that the edge dam be in contact with the web, and wear or abrasion of the edge dam or marking and tearing of the web may occur. Consequently, the edge dam is only positioned closely adjacent to, but spaced from, the web, and some coating liquid seeps therepast.
In use of edge dams, an effort is made to minimize leakage of coating liquid. However, it is usually very difficult, if not impossible, to mount edge dams at the side edges of the application zone in sufficiently close proximity to the paper web to allow only an acceptably small amount of leakage. In practice, some coating liquid leaks past the edge dam, with the result that not only do web edges become contaminated by stickers of coating liquid which bond together adjacent edges of a wound paper roll and prevent the roll from later being properly unwound, but also the coating liquid at the edges is carried by the web and deposited on rolls following or downstream from the coater station.
If leakage of coating liquid past edge dams could be eliminated, it would then be possible to coat only the center portion of a paper web, leaving narrow uncoated strips along opposite side edges of the web. This would advantageously reduce or eliminate coating material buildup on backing rolls following the coater as well as edge stickers on the coater backing roll and web. Unfortunately, conventional edge dams do not ordinarily permit the foregoing to be accomplished.
A significantly improved edge dam assembly which minimizes the above mentioned disadvantages is taught in U.S. Pat. No. 4,327,662, issued to Wayne A. Damrau on May 4, 1982 and assigned to the assignee of the present invention. As disclosed therein, an edge dam assembly is characterized by a seal element sealed with a front wall of a chamber at a forward end of an application zone and a doctor blade at a trailing end of the zone in an end space therebetween and extending toward and closely adjacent to but spaced from the web. The face of the seal element adjacent the paper web has a plurality of grooves extending generally parallel to the direction of movement of the web, so that coating material which seeps past an inner end of the seal element enters and gravitates through the grooves away from the web. In consequence, the amount of coating material leaking past the seal element and deposited on the web edge is significantly minimized. However, and while substantial improvements are obtained, it occasionally happens that coating material reaches and is deposited on the web edge.